"Trump is not only a problem for the EU but also for the whole world," Schulz said in an interview with Der Spiegel published online on Thursday.
"If a man who shows off by not having a clue and by saying that specialist knowledge is elitist rubbish ends up in the White House, a critical point will have been reached," said Schulz, a German center-left Social Democrat.
Such a scenario would mean that there is an "obviously irresponsible man" sitting in a position that demands the utmost sense of responsibility, Schulz said, Reuters reported.
He added that he was concerned that Trump getting into power would spur on copycats in Europe. "That's why I want Hillary Clinton to win."